As methods for preparing compound metal oxides, there have been processes in which
(1) Oxides, carbonates or oxalates of the respective component metals are physically mixed in the state of powder, the mixture is fired and pulverized;
(2) Water-soluble salts such as nitrates of the respective components metals are dissolved in water to make a homogeneous solution, the metals are coprecipitated as insoluble oxalates by addition of oxalate ions, and the precipitate is collected and fired into a compound metal oxide, etc.
In process (1), powders are mixed and fired. Therefore, the reaction proceeds from the contact boundaries of the powder particles and there remain some unreacted portions. Thus the product often deviates in compositions from that intended.
In process (2), the condition of precipitation differs from metal to metal, and, therefore, a precipitate containing a rare earth metal, an alkaline earth metal and copper, for instance, in the content ratio exactly the same as the content ratio in the solution is not always formed. Therefore, compound metal oxides of compositions different from that intended are inevitably formed.
In process (2), a method in which precipitation of oxalate by addition of oxalate ions is caused at the pH of around 5 is is preferably employed. Even in this method, however, sometimes compound metal oxides the composition of which deviates by several tens % from the intended composition are produced, and thus a compound metal oxide of the intended composition cannot be produced regularly and reproducibly.
This invention relates to an improvement of the above-described conventional coprecipitation process. Disadvantages of conventional coprecipitation processes are diminished by slightly basifying a solution containing a predetermined content ratio of chlorides, nitrates or acetates of a rare earth metal, an alkaline earth metal and copper to form hydroxides of the rare earth metal and copper at first, and then introducing carbon dioxide gas into the thus formed suspension to form carbonate of the alkaline earth metal, that is, by forming a mixture of hydroxides of the rare earth metal and copper and carbonate of the alkaline earth metal.